Justin Bieber

Followers: 8.4 million
Sample tweet: "MANCHESTER was one of the best shows ever. It was just FUN!! CROWD WAS HYPED!! THANK YOU and LET'S Do It Again Tomorrow!! #myworldtour"

Earlier generations of obsessed young female fans had to rely on fanzines and weekly tabloids to follow their celebrity crushes; today's screaming tweens can follow their heartthrob's every move on Twitter. From a cheerful "Good morning!" when he awakes to a "Good night" as he tucks himself into bed, Justin Bieber tweets just about everything. But his continuous stream of messages only highlights his unprecedented ability to connect with his fans. Fitting for the teen who was made famous by YouTube, the singer tweets at his followers, replies to questions and follows fans upon their request  unlike other celebs who only reluctantly engage with their fans online. He has 8.4 million followers  making him the second most popular user on the site  but he also follows more than 100,000 people (a rare occurrence among the famous). Perhaps the greatest sign of his Twitter dominance? The company was forced to tweak its trending-topic formula to focus more on phrases of the moment rather than the most consistently popular topics, in an effort to ensure Bieber's name would no longer be an immovable fixture on the site's home page. (Proof that you can't keep 8 million teenagers down: Bieber's ever crafty fans started inventing words like Jieber and Twieber in order to fool the algorithm and keep their idol in the rankings.)